1. Field of the Invention
The present invention relates to a suspension system of an automotive vehicle, and more particularly to an electric control apparatus for controlling a damping coefficient of a damper device in the suspension system.
2. Description of the Prior Art
In Japanese Patent Laid-open Publication No. 5(1993)-294122, there has been proposed an electric control apparatus for a damper device in a suspension system of an automotive vehicle, wherein absolute velocity x' of a sprung mass of the vehicle in vertical movement and relative velocity y' of the sprung mass to an unsprung mass of the vehicle in vertical movement are detected to determine a target damping coefficient of the damper device in a small value when the detected absolute velocity x' is different from the relative velocity y' in a movement direction (when vibration of the sprung mass is in a vibration region) and to increase the target damping coefficient of the damper device in accordance with an increase of a velocity ratio x'/y' of the detected absolute velocity x' to the detected relative velocity y' when both the absolute velocity x' and relative velocity y' are coincident in the movement direction (when vibration of the sprung mass is in a damping region).
In the control apparatus described above, both the absolute velocity x' and relative velocity y' will change sinusoidal with a large phase difference. Particularly, when the sprung mass of the vehicle is vibrating at a low resonance frequency of about 1 Hz, the phase difference becomes approximately ninety degrees, and the velocity ratio x'/y' of the absolute velocity x' to the relative velocity y' changes sinusoidal as shown in FIG. 6. This means that the damping coefficient of the damper device suddenly increases when both the absolute velocity x' and relative velocity y' coincides in the movement direction. Such a sudden change of the damping coefficient will occur as well as at start of vibration of the sprung mass and deteriorate the riding comfort of the vehicle when the sprung mass was vibrated.